. Bulletin of the British Museum (Natural History). 264 TAYLOR, KENNEDY AND HALL TRIDACNACEA (Plate 3, figs 5-8; text-fig. 9) This is a small superfamily closely related to the Cardiacea (Stasek, 1962); included genera are Tridacna and Hippopus. Four species have been examined structurally and mineralogically. The shell is wholly aragonitic. The shell is very thick, with t\\'o shell layers, an outer crossed-lamellar laj'cr and an inner complex crossed-lamellar layer which is bounded in extent by the trace of the pallial line. In the outer layer the first order lamels are large (Plate 3, fig. 6
. Bulletin of the British Museum (Natural History). 264 TAYLOR, KENNEDY AND HALL TRIDACNACEA (Plate 3, figs 5-8; text-fig. 9) This is a small superfamily closely related to the Cardiacea (Stasek, 1962); included genera are Tridacna and Hippopus. Four species have been examined structurally and mineralogically. The shell is wholly aragonitic. The shell is very thick, with t\\'o shell layers, an outer crossed-lamellar laj'cr and an inner complex crossed-lamellar layer which is bounded in extent by the trace of the pallial line. In the outer layer the first order lamels are large (Plate 3, fig. 6) and arranged concentrically in all but the hinge area. The strong ribbing however causes an apparent complex pattern of first order lamels (Plate 3, fig. 4). There is a thin prismatic pallial myostracum in all the species examined. The inner complex crossed-lamellar layer is somewhat variable in character; in the three species of Tridacna studied the structural elements are fairly coarse and interleaved with thin sheets of myostracal-prisms. In Hippopus however the structure is very fine with an almost homogeneous appearance and with many fine prismatic sheets (Plate 3, figs 7 & 8). Higher magnifications (Plate 3, fig. 7) show that the structure consists of sheets of fine needles. All species show very strong daily growth bands in both layers and show prismatic pedal and adductor Fig. g. Radial section of Tridacna maxima. CL = crossed-lamellar, CCL complex crossed- lamellar, PM = pallial Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original British Museum (Natural History). London : BM(NH)
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